This invention relates to a method for forming a thin film and particularly to a method for forming a compound thin film which is deposited on a substrate by means of reactive ionized cluster beam method (R-ICB method).
Conventionally, a compound thin film such as TiN, A1.sub.2 0.sub.3, or SiC, is deposited on the surface of a substrate by sputtering, chemical vapor deposition (CVD) or the like. Even using these methods, however, there were problems in that the thin film deposited on the surface of the substrate had insufficient hardness and low adhesive strength with the substrate. For solving these problems, the reactive-ionized cluster beam (R-ICB) method was developed, in which the thin film is formed by emitting the vapor of a material to be deposited into a reactive gas atmosphere.
FIG. 1 is a schematic view showing an R-ICB apparatus which is disclosed, for example, in Japanese Pat. Publication No. 57-54930 or Proceedings of the International Ion Engineering Congress (ISIAT '83 and IPAT '83). In FIG. 1, an evacuation system 5 evacuates gases in a vacuum chamber 6 and maintains the pressure in the chamber 6 at a predetermined value. A reactive gas introduction system for introducing reactive gases into the chamber 6 comprises a gas cylinder 41 for charging reactive gases such as oxygen, nitrogen and hydrocarbons into the chamber 6, a flow regulation valve 42 for regulating the flow rate of the reactive gases which are supplied from the gas cylinder 41 to the chamber 6, and a gas introduction pipe 43 for introducing the reactive gases to a predetermined portion of the chamber 6. A vapor generating means 1 for generating a vapor of a material to be deposited comprises a closed type crucible 12 having a nozzle 11, in which a material 15 to be deposited is packed a coiled filament 13 disposed around the crucible 12 for heating the crucible 12 to evaporate the material 15 therein, and a heat shielding plate 14 for shielding the heat of the filament 13. Vapor of the material 15 is emitted through the nozzle 11 of the crucible 12 to form clusters 16 consisting of a large number of atoms. The clusters 16 formed are ionized by an ionization means 2 which comprises an electron beam emitting means such as a coiled filament 21, an electron beam extraction electrode 22 for extracting electrons from the filament 21 and accelerating them, and a heat shielding plate 23 for shutting out the heat of the filament 21. An acceleration electrode 3 disposed over the ionization means 2 accelerates the ionized clusters 16a by its electrical field, so as to impart more kinetic energy to the ionized clusters 16a. Disposed opposite the acceleration electrode 3 is a substrate 7 on the surface of which a compound thin layer 71 is to be formed. An electric power unit 8 for the vapor generating means 1 is provided on the outside of the chamber 6 and contains direct current power sources 81, 82 and 83 for applying bias voltages and power sources 84 and 85 for heating the filaments 13 and 21. Each of the bias power sources in the power unit 8 functions as follows. The first direct current power source 81 biases the potential of the crucible 12 positive in respect to the filament 13 so that thermal electrons emitted from the filament 13, heated by using the power source 84, collide with the crucible 12. The second direct current power source 82 biases the potential of the filament 21, heated by the power source 85, negative in respect to the electrode 22, thereby drawing the thermal electrons emitted from the filament 21 into the inside of the electrode 22. The third direct current power source 83 biases the potentials of the electrode 22 and the crucible 12 positive in respect to the acceleration electrode 3 which is at ground potential, to control the acceleration of the positive charged cluster ions by means of the electrical field lens formed between the electrode 22 and the crucible 12.
The conventional R-ICB apparatus is constructed as mentioned above operates as follows. After evacuating the chamber 6 by the evacuation system 5 to a pressure of around 1.times.10.sup.-6 mmHg, the reactive gases are introduced into the chamber 6 through the pipe 43 by opening the valve 42. A portion of the reactive gases introduced into the chamber 6 is activated, namely excited or dissociated, at a location above and near over the vapor generating means 1. Then, this portion of the reactive gases thus partially activated reaches the surface of the substrate 7.
On the other hand, the crucible 12 is heated by a collision of the electrons emitted from the filament 13 to which direct current is applied by means of the power source 81. By the heating of the crucible 12, the material 15 in the crucible 12 is vaporized to emit the vapor therefrom through the nozzle 11 into the chamber 6. At this time, the crucible 12 is heated up to a temperature where the vapor pressure of the material 15 in the crucible 12 reaches several mmHg. When the vapor emitted from the crucible 12 passes through the small nozzle 11, the clusters 16 are formed by adiabatic expansion of the vapor due to the pressure differences between the crucible 12 and the chamber 6 under supercooling conditions. The clusters 16 are massive atomic groups formed by condensing 100 to 1000 atoms of the material 15 under the above conditions. A portion of the clusters 16 are then ionized by the electrons emitted from the filament 21 to make the cluster ions 16a. The cluster ions 16a and non-ionized clusters 16 are accelerated by the electrical field generated by the acceleration electrode 3 and drift toward the substrate 7. Therefore, the substrate 7 will be surrounded by the reactive gases introduced into the chamber 6. Then, the reaction of the cluster ions 16a and clusters 16 with the reactive gases occurs in the neighborhood of the substrate 7 to produce compounds. A thin film 71 of the compounds produced is then deposited on the substrate 7.
In the above-mentioned conventional method and apparatus for forming the thin film of compounds, the reactive gases in the chamber 6 are in a molecular state and the activity thereof is low. Moreover, since the reactive gases activated around the vapor generating means 1 have short activity spans, the activated reactive gases near the substrate 7 are apt to return to a low activity condition. Therefore, the reactivity of the compounds formed by the reaction is low, making the thin film deposited on the substrate 7 unstable. Further, most of the reactive gases are evacuated without being used to form the thin film of the compounds.